1. Field of the Invention
The present invention relates to process control systems and more particularly to arrangements for supplying backup control signals to field control devices when their normal control signals must be interrupted during servicing and maintenance.
2. Description of the Prior Art
In known process control systems, sensing signals are supplied from field sensing devices to a central data processor, which in turn supplies field control devices with control signals appropriate to maintain the process running in a desired manner. In the event of a failure in the circuits supplying control signals to the field devices or during routine maintenance, the control signals must be interrupted for servicing. In order to supply a backup signal during interruptions, process control systems have been supplied with a portable maintenance unit carried to the location where service is necessary and connected into the circuitry to supply a backup signal for as long as it takes to service the circuit. In many process control systems, however, the site where service is required is often far away from the central control station where the computer and display and indicator devices are installed to provide data showing how the process is, and ought to be, proceeding. In order to keep the process running in the desired manner, such data must be used to periodically modify and update the backup control signal. Moreover, the need for service is liable to arise anywhere in the control system, requiring the portable maintenance device to be moved from place to place. This has slowed servicing, and impaired process efficiency.
Because of the foregoing problems, known process control systems have not been entirely satisfactory in performing a maintenance function.